I am using Windows 8.1. I had the same problem.
I installed gulp using Node.js command prompt
npm install -g gulp
Then go to the required directory in Node.js command prompt and try
gulp -v
If you get gulp local version not found exit the current Node.js command prompt and try the above command in a new Node.js command prompt
I tried the NODE_PATH mentioned by @SteveLacy but the command prompt was still not able to detect gulp command